JP4060551B2 - Optical signal transmission apparatus and optical signal transmission method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to, for example, a transmission device in a Faraday mirror type quantum cryptography device.
[0002]
[Prior art]
FIG. Ribordy, et. al. “Automated 'plug &play' quantum key distribution,” ELECTRONICS LETTERS Vol. 34 No. 22 pp. 2116-2117 (1998) or International Patent Publication No. WO98 / 10560 QUANTUM CRYPTOGRAPHY DEVICE AND METHOD is a configuration diagram of a conventional Faraday mirror type quantum cryptography device. In FIG. A coupler 1 connected to the optical fiber 10; a photodetector 2 that detects a light pulse input to the coupler 1 from the communication optical fiber 10; a polarization controller 3 that adjusts a polarization mode of the input light pulse; and an intensity of the light pulse. Attenuator 4 that reduces the intensity of the optical pulse output from the quantum cryptography device to a quantum level (0.1 photon per pulse), reflects the input pulse by rotating its polarization plane by 90 degrees, Therefore, the input pulse of TE polarization is the optical pulse of TM polarization. Reflected as the input pulse of the TM polarization and a phase modulator 8 for applying a phase modulation to the Faraday mirror 7, passes the pulse reflected as the light pulse of the TE polarization. The quantum cryptography receiver 200 includes a coupler 51, a photon detector 52, a photon detector 53, a polarization controller 54, a polarization controller 55, a polarization beam splitter 56, a circulator 57, a phase modulator 58, a control plate 59, a laser 60, and a short optical path. 61, a long optical path 62.
[0003]
Next, the operation will be described with reference to FIG. The quantum cryptography receiver 200 of FIG. 7 generates an optical pulse P by the laser 60. The optical pulse P is split into a short optical path 61 and a long optical path 62 by the coupler 51. The polarization direction of the light pulse in the long optical path 62 is adjusted by the polarization controller 55, passes through the phase modulator 58, and is output to the communication optical fiber 10 by the polarization beam splitter 56. An optical pulse in the short optical path 61 is also output to the communication optical fiber 10. Since the long optical path 62 has a longer path than the short optical path 61, two different pulses P1 and P2 are output to the communication optical fiber 10. Thus, the optical pulses P1 and P2 having two different polarization modes are output to the communication optical fiber 10.
[0004]
Optical pulses P1 and P2 having two different polarization modes are input to the quantum cryptography transmission device 100 through the communication optical fiber 10 at different timings. The optical pulses P1 and P2 input through the communication optical fiber 10 are each divided into two by the coupler 1, and one optical pulse P1, P2 divided by the coupler 1 is detected by the photodetector 2. According to the optical pulse detection timing of the optical detector 2, the phase modulator 8 modulates only the optical pulse P2 of the optical pulses P1 and P2. The polarization planes of the other optical pulses P1 and P2 divided by the coupler 1 are adjusted by the polarization controller 3 so that the phase modulator 8 works optimally. At this time, the first optical pulse P1 out of the two optical pulses P1 and P2 input to the quantum cryptography transmission device 100 at different timings is adjusted so as to be in the TE polarization mode. Therefore, the second optical pulse P2 becomes a TM polarization mode. An optical pulse that passes through the polarization controller 3 and the attenuator 4 and travels toward the Faraday mirror 7 passes through the phase modulator 8 and is input to the Faraday mirror 7. The optical pulse input to the Faraday mirror 7 is reflected as a TM-polarized light pulse when the polarization mode is a TE-polarized light pulse, and the TM-polarized light pulse is reflected as a TE-polarized light pulse. . The reflected light pulse passes through the phase modulator 8 again. The timing of the phase modulator 8 is adjusted by the control plate 9 so that only the second optical pulse P2 of the two optical pulses P1 and P2 reflected by the Faraday mirror 7 and passing through the phase modulator 8 is subjected to phase modulation. Has been. The phase-modulated optical pulse P2 is transmitted so as to reverse the optical path that has entered the communication optical fiber 10. The two optical pulses P 1 and P 2 that have passed through the phase modulator 8 after being reflected by the Faraday mirror 7 are directed to the attenuator 4. The attenuator 4 attenuates the intensity of the optical pulse until the intensity of the optical pulse subjected to phase modulation by the phase modulator 8 reaches a quantum level (0.1 photons per pulse). Thereafter, the light pulse passes through the polarization controller 3 and the coupler 1 in this order and is transmitted to the communication optical fiber 10.
[0005]
[Problems to be solved by the invention]
In the conventional Faraday mirror type quantum cryptography transmission device, since the optical pulse input to the device passes the same optical path in the forward path and the return path, the optical pulse passes through the phase modulator 8 twice and the polarization mode is Since the light passes through both the TE polarization mode with relatively little loss and the TM polarization mode with extremely large loss, the loss L of the light intensity becomes very large. When adjusting the quantum cryptography device, the attenuator 4 is removed to adjust each part in order to increase the S / N ratio (signal / noise ratio). If the loss L of the light intensity is large, the S / N ratio at the time of quantum cryptography device adjustment is increased. There was an extremely small problem.
[0006]
Here, the loss of light intensity will be described.
For example, the intensity of the optical pulse input from the communication optical fiber 10 is S, the TE polarization loss of the phase modulator 8 is L8 (TE), and the TM polarization loss of the phase modulator 8 is L8 (TM). Suppose that other losses are LZ and take the following values.
S = 50 dB
L8 (TE) = 6dB
L8 (TM) = 30dB
LZ = 2dB
If the loss of the entire light intensity is L, L can be obtained by the following equation.
L = L8 (TE) + LZ + L8 (TM) + LZ
= 6 + 2 + 30 + 2
= 40 dB
If the intensity of the light pulse when adjusting by removing the attenuator 4 is M,
M = S−L = 50−40 = 10 dB
Thus, the greater the loss L, the smaller the intensity M of the light pulse, the worse the S / N ratio, and the adjustment work becomes difficult.
[0007]
It is an object of the present invention to provide a quantum cryptography transmission device with a small light intensity loss during quantum cryptography adjustment.
[0008]
[Means for Solving the Problems]
An optical signal transmission device according to the present invention receives an optical signal, becomes an optical path of the optical signal, and a first optical path for transmitting the optical signal;
First and second polarization beam splitters provided in the first optical path, for separating an optical signal from the first optical path;
A second optical path provided between the first and second polarizing beam splitters and serving as an optical path of an optical signal separated by the first and second polarizing splitters;
And a phase modulator provided on the second optical path for applying phase modulation to the optical signal.
[0009]
The optical signal transmitter further includes:
A mirror for changing the polarization mode of the optical signal and reflecting the optical signal at an end of the first optical path;
The first optical path is used as an optical signal forward path and return path,
The second optical path is used as a forward path and a return path of an optical signal separated by the first and second polarization beam splitters.
[0010]
The first optical path receives an optical signal having a TE-polarized optical pulse and a TM-polarized optical pulse,
The first and second polarization beam splitters separate TE-polarized light pulses,
The phase modulator performs phase modulation on a TE-polarized optical pulse.
[0011]
An optical signal transmission method according to the present invention includes:
A separation step of separating the TE polarized light into the second optical path from the optical signal having the TE polarized light and the TM polarized light flowing through the first optical path;
A phase modulation step of applying phase modulation to the TE polarized light separated into the second optical path by the separation step;
And a merging step of merging the TE-polarized light subjected to the phase modulation by the phase modulation step into the first optical path.
[0012]
The optical signal transmission method includes an outward path step and a backward path step of reflecting an optical signal to reciprocate an optical path,
The phase modulation process is performed in a return path process.
[0013]
An optical signal transmission device according to the present invention receives an optical signal, becomes an optical path of the optical signal, and transmits / receives an optical signal;
A polarization beam splitter provided at an end of the transmission / reception optical path and separating an optical signal from the transmission / reception optical path;
A loop optical path that is connected to the polarizing beam splitter and is an optical path for circulating the optical signal separated by the polarizing beam splitter to the polarizing beam splitter;
A phase modulator that is provided in the loop optical path and applies phase modulation to the optical signal;
And a polarization mode changer that is provided in the loop optical path and changes a polarization mode of the optical signal.
[0014]
The polarization mode changer includes a fast / slow coupler that changes the polarization mode by connecting the fast axis and slow axis of the polarization axis of the optical fiber.
The transmission / reception optical path is used as the forward path and the return path of the optical signal,
The loop optical path is used as a forward path and a return path of an optical signal separated by a polarization beam splitter.
[0015]
The transmission / reception optical path receives an optical signal having a TE-polarized light pulse and a TM-polarized light pulse, and the polarization beam splitter separates the TE-polarized light pulse and the TM-polarized light pulse. The phase modulator performs phase modulation on a TE-polarized optical pulse.
[0016]
The optical signal transmission method according to the present invention separates TE polarization and TM polarization from an optical signal that flows through a transmission / reception optical path and has TE polarization and TM polarization, and outputs it to one end and the other end of a loop optical path. A separation process;
A phase modulation step of applying phase modulation in the loop optical path to the TE polarized wave separated by the separation step;
And a merging step for merging optical signals output from the other end and one end of the loop optical path.
[0017]
The optical signal transmission method includes an outward path step for reciprocating an optical signal in a transmission / reception optical path, a return path step, and a circulation step for allowing the optical signal to flow in a loop optical path,
The phase modulation step is performed in a reflux step.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
Embodiment 1 FIG.
FIG. 1 is an optical system configuration diagram showing a quantum cryptography transmission device 100 in a Faraday mirror quantum cryptography device. The Faraday mirror type quantum cryptography transmitter according to this embodiment uses two polarization beam splitters to make the forward path and the return path of the optical path in the transmitter different.
In the figure, a quantum cryptography transmission device 100 includes a coupler 1 connected to a communication optical fiber 10, a photodetector 2 that detects an optical pulse input from the communication optical fiber 10 to the coupler 1, and a polarization mode of the input optical pulse. The polarization controller 3 for adjusting the intensity of the optical pulse, attenuator 4 for attenuating the intensity of the optical pulse and setting the intensity of the optical pulse output from the quantum cryptography device to the quantum level (0.1 photon per pulse), and the polarization mode of the optical pulse. Accordingly, the TE-polarized light pulse is input to the polarization beam splitter 5 and the polarization beam splitter 6, which are automatically switched to the modulation optical path 13 that passes through the phase modulator 8 and the bypass optical path 11 that bypasses the TM-polarized light pulse. The pulse is reflected by rotating its plane of polarization 90 degrees, so the TE polarized input pulse is reflected as a TM polarized optical pulse. , The input pulse of the TM polarization and a phase modulator 8 for applying a phase modulation to the Faraday mirror 7, passes the pulse reflected as the light pulse of the TE polarization. The optical path connecting the attenuator 4, the polarizing beam splitter 5, the polarizing beam splitter 6, and the Faraday mirror 7 is the first optical path R1. The optical path connecting the polarizing beam splitter 5, the phase modulator 8, and the polarizing beam splitter 6 is the second optical path R2. The second optical path R2 is arranged in parallel with the first optical path R1. The phase modulator 8 is disposed in the second optical path R2. Other configurations are the same as those in FIG.
[0019]
Next, the operation will be described with reference to FIGS.
FIG. 2 is an operation flowchart of the quantum cryptography transmission device. FIG. 3 is a state diagram in each part of the optical pulse. FIG. 4 is a diagram illustrating a time-series passage state of optical pulses in the bypass optical path 11 and the modulation optical path 13. 3 and 4, P, P1, and P2 indicate pulses. Further, the arrows L4, L5, L6, and L8 described at the top of each pulse indicate that there is a loss of light intensity due to the attenuator 4, the polarizing beam splitter 5, the polarizing beam splitter 6, and the phase modulator 8, respectively.
[0020]
(1) Outbound process S20
First, optical pulses P1 and P2 having two different polarization modes are input to the quantum cryptography transmission device of FIG. 1 at different timings through the communication optical fiber 10 (S1). The optical pulses P1 and P2 input through the communication optical fiber 10 are each divided into two by the coupler 1, and one optical pulse P1 and P2 divided by the coupler 1 is detected by the photodetector 2. According to the optical pulse detection timing of the optical detector 2, the phase modulator 8 modulates only the optical pulse P2 of the optical pulses P1 and P2. The polarization planes of the other optical pulses P1 and P2 divided by the coupler 1 are adjusted by the polarization controller 3 so that the phase modulator 8 works optimally (S2). At this time, the first optical pulse P1 out of the two optical pulses P1 and P2 input to the quantum cryptography transmission device at different timings is adjusted so as to be in the polarization mode of TE polarization. Therefore, the second optical pulse becomes a TM polarization mode. Next, the intensity of the light pulse is weakened by the attenuator 4 (S3). An optical pulse that passes through the polarization controller 3 and travels toward the Faraday mirror 7 is transmitted by the polarization beam splitter 5, and an optical pulse P 1 whose polarization mode is TE-polarized light is transmitted to the modulated optical path 13 that passes through the phase modulator 8. The pulse P2 is selected for the bypass optical path 11 directly toward the polarization beam splitter 6 (S4). The two optical pulses P1 and P2 that have passed through different optical paths are combined by the polarization beam splitter 6 and input to the Faraday mirror 7 (S5). The optical pulse input to the Faraday mirror 7 is reflected as a TM-polarized light pulse P1 when the polarization mode is a TE-polarized light pulse, and the TM-polarized light pulse is reflected as a TE-polarized light pulse P2. (S6).
[0021]
(2) Return path process S30
The reflected light pulses P1, P2 are polarized beam splitter 6, and the TE polarized light pulse P2 passes through the phase modulator 8 according to the polarization mode, and the TM polarized light pulse P1 is directly polarized by the polarization beam splitter. Is selected as the bypass optical path 11 directed to 5 (S7). The timing of the phase modulator 8 is adjusted by the control plate 9 so as to apply phase modulation only to the optical pulse P2 reflected by the Faraday mirror 7 and passing through the phase modulator 8 (S8). The optical pulse P1 that has not undergone phase modulation and the optical pulse P2 that has undergone phase modulation are transmitted toward the optical fiber for communication 10 so as to reverse the incident optical path. The two optical pulses P1 and P2 that have passed through the different optical paths after being reflected by the Faraday mirror 7 are merged by the polarization beam splitter 5 and directed to the attenuator 4 (S9). The attenuator 4 attenuates the intensity of the optical pulse until the intensity of the optical pulse subjected to phase modulation by the phase modulator 8 reaches a quantum level (0.1 photons per pulse) (S10). Thereafter, the light pulse passes through the polarization controller 3 and the coupler 1 in this order, and is transmitted to the communication optical fiber 10 (S11).
[0022]
As shown in FIG. 4, the optical pulse passing through the bypass optical path 11 which is a part of the first optical path R1 is only the TM-polarized optical pulse. On the other hand, the optical pulse passing through the modulation optical path 13 of the second optical path R2 is only a TE-polarized optical pulse. The order in which these light pulses pass is the order of arrows A1, A2, and A3 in FIG. Moreover, it is the order of arrow A4, A5, A6.
[0023]
Here, the loss of light intensity will be described.
For example, the intensity of the optical pulse input from the communication optical fiber 10 is S, the intensity loss of the optical pulse by the polarization beam splitter 5 is L5, the intensity loss of the optical pulse by the polarization beam splitter 6 is L6, and the phase modulator The loss of the intensity of the optical pulse due to 8 is L8, and the other loss is LZ, and the following values are taken.
S = 50 dB
L5 = 5dB
L6 = 5dB
L8 = 6dB
LZ = 2dB
If the loss of the entire light intensity is L, L can be obtained by the following equation.
L = (L5 + L6) + LZ + (L6 + L8 + L5) + LZ
= 5 + 5 + 2 + 5 + 6 + 5 + 2
= 30 dB
As described above, the optical pulses incident on the transmission device include two optical pulses of TE polarization and TM polarization for the phase modulator 8. The optical pulse is reflected by the Faraday mirror 7 and the TE polarized wave is reflected by the TM polarized wave, and the TM polarized wave is reflected by the TE polarized wave having its polarization plane rotated. Conventionally, the phase modulator 8 is passed through the optical modulator in two states of TE polarization and TM polarization. However, the phase modulator has a low transmittance with respect to the TM polarization, and in the conventional configuration, the incident pulse is output with a reduction of 40 dB, for example.
However, in this embodiment, two polarization beam splitters 5 and 6 polarization beam splitters 6 are used, and TM-polarized light pulses bypass the phase modulator 8. Only the TE-polarized light pulse is passed through the phase modulator 8. Thus, the reduction of the incident pulse is suppressed to 30 dB, and an improvement of 10 dB is achieved in terms of the S / N ratio.
[0024]
As described above, in this embodiment, the optical path in the quantum cryptography transmitter is provided separately for the forward path and the return path using the two polarization beam splitters 5 and the polarization beam splitter 6, and the phase modulator is provided on one of the optical paths. The Faraday mirror type quantum cryptography transmission device characterized in that 8 is installed has been described.
[0025]
In this embodiment, the optical pulse passing through the quantum cryptography transmission device is separated from the forward path and the return path by the two polarization beam splitters 5 and 6 and passes through the phase modulator 8 only once. Since it passes only in the polarization mode of the wave, the loss of the incident pulse by the quantum cryptography transmitter is 30 dB when the attenuator 4 is removed, and an improvement of 10 dB can be realized as compared with the loss of the quantum cryptography transmitter of the prior art. Therefore, at the time of adjustment, the S / N ratio is converted to 10 dB, and the quantum cryptography apparatus can be adjusted more easily.
[0026]
Embodiment 2. FIG.
In FIG. 1, the polarization beam splitter 5 and the polarization beam splitter 6 that reflect the TE polarization and pass the TM polarization are used. However, as shown in FIG. A polarizing beam splitter 5a that reflects and a polarizing beam splitter 6a that passes TE polarized light may be used.
[0027]
Further, as shown in FIG. 6, a polarization beam splitter 5 that passes TM polarization and a polarization beam splitter 6a that passes TE polarization may be used. Alternatively, although not shown, a polarization beam splitter 6 that passes 5a that passes TE polarization and TM polarization that passes TM may be used.
[0028]
Further, although the Faraday mirror 7 is used in FIG. 1, other than the Faraday mirror 7 may be used as long as it has the same function as the Faraday mirror 7.
[0029]
Embodiment 3 FIG.
FIG. 9 is a diagram showing a configuration in which the Faraday mirror 7 is not used.
In FIG. 9, the transmission device includes a transmission / reception optical path R3 and a loop optical path R4. A polarization controller 3, an attenuator 4, and a polarization beam splitter 5 are provided in the transmission / reception optical path R3. The polarization beam splitter 5 has three ports A, B, and C. The A port connects the transmission / reception optical path R3. The B port connects one end of the loop optical path R4. The C port connects the other end of the loop optical path R4. With this configuration, the optical signal output from the B port is input to the C port. The optical signal output from the C port is input to the B port. The looping of the optical signal between the B port and the C port using the loop optical path R4 in this way is hereinafter referred to as reflux.
A phase modulator 8 and a fast / slow coupler 70 are provided in the loop optical path R4. The first / slow coupler 70 changes the TM polarization to the TE polarization and connects the TE polarization to the TM polarization by connecting the first axis and the slow axis of the polarization axis of the optical fiber. The first / slow coupler 70 is an example of a polarization mode changer.
The polarization beam splitter 5 is used to separate the TM-polarized light pulse and the TE-polarized light pulse, and the TE-polarized light pulse is directly passed through the phase modulator 8. The TM-polarized light pulse passes through the first slow coupler 70 to the other port of the phase modulator 8.
[0030]
FIG. 10 is an operation flowchart of the quantum cryptography transmission device shown in FIG.
(1) Outward process S40
The operations of S1 to S4 in the forward step S40 shown in FIG. 10 are the same as the operations of S1 to S4 shown in FIG.
(2) Circulation process S50
The TE-polarized light pulse separated by the polarization beam splitter 5 is input to the phase modulator 8 and undergoes phase modulation (S8). Next, the TE-polarized optical pulse subjected to the phase modulation is input to the first / slow coupler 70, the polarization mode is changed, and the TM-polarized optical pulse is output.
On the other hand, the TM-polarized light pulse separated by the polarization beam splitter 5 is input to the first / slow coupler 70, changed from TM-polarized light to TE-polarized light, and output. The TE-polarized optical pulse output from the first / slow coupler 70 is input to the phase modulator 8, but is not subjected to phase modulation and is output to the polarization beam splitter 5 as it is.
(3) Return path process S60
The operations of S9 to S11 in the return path step S60 of FIG. 10 are the same as the operations of S9 to S11 shown in FIG.
[0031]
The forward path step S40 and the backward path step S60 described above are operations performed in the transmission / reception optical path R3. Further, the above-described circulation step S50 is an operation performed in the loop optical path R4.
[0032]
Even when the configuration shown in FIG. 9 is used, the TE-polarized optical pulse output from the B port passes through the phase modulator 8 only once and is returned to the C port. Therefore, the loss of light intensity is minimized, and the same effect as that of the above-described embodiment can be obtained.
[0033]
The first / slow coupler 70 described above is an example of a polarization mode changer, and other devices may be used as long as the change between TM polarization and TE polarization is possible. . For example, a ½λ plate (λ is a wavelength) may be used. Alternatively, the optical communication cable may be twisted 90 degrees. Alternatively, the optical communication cables may be connected 90 degrees orthogonal to each other.
[0034]
【The invention's effect】
As described above, according to the Faraday mirror quantum cryptography transmission device of the preferred embodiment of the present invention, the optical path in the device is divided between the forward path and the return path, and the optical pulse passing through the phase modulator 8 once. Therefore, there is an effect that the strength loss can be reduced, the S / N ratio at the time of adjusting the quantum cryptography transmission device can be increased, and the adjustment becomes easy.
[0035]
Further, according to the present invention, since the loop optical path is used, there is an effect that the Faraday mirror is not used and the configuration of the apparatus is simplified.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of an optical system of a Faraday mirror type quantum cryptography transmission device according to a preferred embodiment of the present invention;
FIG. 2 is an operation flowchart of FIG.
FIG. 3 is a diagram showing a state of an optical pulse.
FIG. 4 is a diagram illustrating a time-sequential passage state of optical pulses.
5 is a configuration diagram of an optical system according to Embodiment 2. FIG.
6 is a configuration diagram of an optical system according to Embodiment 2. FIG.
FIG. 7 is an overall configuration diagram of a conventional Faraday mirror type quantum cryptography device.
FIG. 8 is a state diagram of an optical pulse in a conventional Faraday mirror type quantum cryptography transmission device.
FIG. 9 is a configuration diagram of an optical system according to a third embodiment.
10 is an operation flowchart of FIG. 9;
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Coupler, 2 Photodetector, 3 Polarization controller, 4 Attenuator, 5 Polarization beam splitter, 6 Polarization beam splitter, 7 Faraday mirror, 8 Phase modulator, 9 Control board, 10 Optical fiber for communication, 11 Bypass optical path, 13 Modulation Optical path, 70 first slow coupler, R1 first optical path, R2 second optical path, R3 transmit / receive optical path, R4 loop optical path.

Claims (5)

A first optical path used as an optical signal forward and return path;
First and second polarization beam splitters provided in the first optical path, for separating an optical signal from the first optical path;
A mirror that changes the polarization mode of the optical signal disposed at the end of the first optical path and reflects the optical signal;
A second optical path provided between the first and second polarizing beam splitters and used as a forward path and a return path of an optical signal separated by the first and second polarizing splitters;
An optical signal transmitting apparatus, comprising: a phase modulator provided in the second optical path for applying phase modulation to an optical signal. The first optical path is used as a forward path and a return path of an optical signal having a TE polarized light pulse and a TM polarized light pulse,
The first and second polarization beam splitters separate TE-polarized light pulses,
2. The optical signal transmitting apparatus according to claim 1, wherein the phase modulator applies phase modulation to a TE-polarized optical pulse. The optical signal transmission device according to claim 1, wherein the second optical path is disposed substantially parallel to the first optical path . A forward path step of transmitting an optical signal having TE polarization and TM polarization to a first optical path used as the forward path and the return path of the optical signal;
A reflection step of reflecting the optical signal transmitted to the first optical path by the forward path step on the mirror disposed at the end of the first optical path;
A separation step of separating the TE polarized light into the second optical path from the optical signal flowing in the first optical path as the return path by reflection in the reflection step;
A phase modulation step of performing phase modulation on the TE polarized light separated into the second optical path by the separation step;
An optical signal transmission method comprising: a merging step of merging TE polarized waves that have been subjected to phase modulation in the phase modulation step into the first optical path. The separation step is
5. The optical signal transmission method according to claim 4, wherein the TE polarized light is separated into a second optical path disposed substantially parallel to the first optical path.

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